Zeolites are crystalline aluminosilicate molecular sieves which have a microporous three-dimensional framework structure. In general, the crystalline zeolites are formed from corner-sharing AlO.sub.2 and SiO.sub.2 tetrahedra and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Zeolites can be represented on an anhydrous basis, by the empirical formula EQU M.sub.2/n O:Al.sub.2 O.sub.3 :XSiO.sub.2
where M is a cation having the valence n, X is generally equal to or greater than 2. In naturally occurring zeolites, M can be Li, Na, Ca, K, Mg and Ba. The M cations are loosely bound to the structure and frequently can be completely or partially replaced with other cations by conventional ion exchange techniques. Currently over 150 species of both naturally occurring and synthetic zeolites are known.
Other crystalline microporous compositions are known which are not zeolitic, i.e., do not contain AlO.sub.2 tetrahedra as essential framework constituents, but which exhibit the ion-exchange and/or absorption characteristics of the zeolites. These include 1) a pure silica polymorph, silicalite, having a neutral framework containing neither cations nor cation sites as disclosed in the U.S. Pat. No. 4,061,724; 2) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440; 3) silicon substituted aluminophosphates as disclosed in U.S. Pat. No. 4,440,871; 4) metal substituted aluminophosphates as disclosed in U.S. Pat. No. 4,853,197; and 5) metal sulfide molecular sieves disclosed in U.S. Pat. No. 4,880,761.
Applicant has discovered a new class of molecular sieves based on tetrahedra of zinc, phosphorus and at least one other metal. These compositions have a three-dimensional microporous framework, a high framework charge and a high charge gradient which arises from the Zn.sup.+2, P.sup.+5 and M'.sup.+n (n=+2, +3 or +4) tetrahedral sites in the framework. This class of compositions are capable of ion exchange, adsorption of gases and are useful as catalysts.
There are a number of reports dealing with the synthesis and characterization of zinc phosphate materials. For example the crystal structure of Zn.sub.2 HK(PO.sub.4).sub.2.2H.sub.2 O has been reported by M. T. Averbuch-Pouchot and A. Durif in J. Applied Crystallography, No. 7, p. 403 (1974). Further, the properties of KZn.sub.2 H(PO.sub.4).sub.2.2.5H.sub.2 O has been reported by T. Barbou des Courieres and M. H. Simonot-Grange in Materials Research Bulletin, Volume 12, pp. 355-360 (1977). In this article the authors report that the zinc phosphate material contains water molecules which have partially zeolitic properties. However, heating of this material at temperatures of about 450.degree. C. results in a collapse of the structure. More importantly, the structure of this material was determined by I. Tordjman et al. in Acta Cryst. B31, 1143 (1975) and showed that two of the zinc atoms in the unit cell were not tetrahedrally coordinated (See Table 4, pp. 1147-48). Therefore, this material does not have a framework made up of tetrahedrally coordinated zinc and phosphorus atoms. Other references include the crystal structure of (Na,K)(ZnPO.sub.4) by O. V. Yakubovich and O. K. Mel'nikov in Sov. Phys. Crystallogr., 34(1) (1989) and A. W. Frazier et al. in J. Agr. Food Chemistry, Volume 14, page 522-529 (1966). However, both of the materials reported in these references are dense phase materials with the former reference stating that their material has a .beta.-tridymite structure.
There are also several references which disclose the synthesis and characterization of microporous zinc phosphate compositions. Gier and Stucky in Nature, 349, 508-510 (1991) disclose the synthesis of zinco (beryllo)-phosphate and arsenate molecular seives. Harrison et al. in Chem. Mater., 3, 27-29 (1991) disclose the preparation of zinc phosphate and beryllium phosphate molecular sieves having the zeolite X structure. The synthesis and characterization of zinc phosphate and beryllium phosphate molecular sieves with the sodalite structure has been reported by Nenoff et al. in J. Am. Chem. Soc., 113, 378-379 (1991). Gier et al. in Angew. Chem. Intl. Ed. Engl., 30 (9), 1169-1171 (1991) disclose and characterize lithium haloberyllophosphate and arsenate molecular sieves.
At this point it is helpful to discuss the difference between "microporous" materials and "dense phase" materials. Microporous materials are materials which have an intracrystalline pore system. The pores are large enough to admit various gaseous or liquid molecules. One example of microporous materials are zeolites. Zeolites contain cations which are found in the intracrystalline pore system of the zeolite and are hydrated. Therefore, a traditional criterion for microporosity is the presence of intracrystalline waters of hydration which are associated with cations found in the pores, i.e., structure directing cations.
Microporous materials are metastable with respect to solid state or dense phases. What this means is that all microporous materials, as a consequence of their metastable nature, will display either a structure collapse or a structure collapse followed by a recrystallization to a dense phase before the melting temperature is reached.
Dense phase materials on the other hand do not have an intracrystalline pore system which is capable of admitting gaseous or liquid molecules. Although in certain crystallographic views, these dense materials may appear to have intracrystalline pore systems, these pores are only large enough to accommodate a small cation without any waters of hydration. That is, the cation is actually coordinated to the framework oxygen atoms. Further, dense phase materials will not undergo a complete structure collapse before melting occurs.
Applicant's compositions differ in several ways from the materials described in the references above. First, applicant's compositions have zinc, phosphorus and one or more other metal in the framework. That is, these compositions have a three-dimensional microporous framework of ZnO.sub.2, PO.sub.2 and M'O.sub.2 tetrahedral units. Illustrative of these M' metals are: magnesium, copper, gallium, aluminum and germanium.
It should be pointed out that molecular sieves containing Al, P and Zn have been disclosed in U.S. Pat. No. 4,567,029. Applicant's compositions differ from those disclosed in the '029 patent in that applicant's compositions contain alkali metals whereas the '029 compositions do not. The amount of aluminum in applicant's composition is outside the range disclosed by the '029 reference. Finally the structure of applicant's composition is different from those disclosed in the '029 reference.